systemd: fall back to mounting /sys/fs/cgroup sans xattr

xattrs on cgroup fs were added back in v3.6-rc3-3-g03b1cde. But we
support kernels >= 2.6.39, and we should also support kernels compiled
w/o xattr support, even if systemd is compiled with xattr support.
Fall back to mounting without xattr support.

Tested-by: Colin Walters <walters@verbum.org>
test: make it easier to override kernel version
Standarize on one spelling of symlink error message

It's polite to print the name of the link that wasn't created,
and it makes little sense to print the target.
Add set_consume which always takes ownership

Freeing in error path is the common pattern with set_put().
readahead: be more verbose about creation failures

systemd-readahead reports "Failed to create shared memory segment:
No such file or directory", but it's unclear how it can happen. Be
more verbose about failures.
Small cleanup
systemctl: show reverse dependencies or before/after ordering

Also update completion scripts a bit.